chore: remove 3.9 from kokoro tests (#1085)

Similar to googleapis/python-firestore#1163 .
Python 3.9 tests will run in Github actions after the migration to
google-cloud-python.

Author: parthea

.librarian/generator-input/noxfile.py

@@ -80,7 +80,6 @@
 CURRENT_DIRECTORY = pathlib.Path(__file__).parent.absolute()
 
 nox.options.sessions = [
-    "unit-3.9",
     "unit-3.10",
     "unit-3.11",
     "unit-3.12",

noxfile.py

@@ -84,7 +84,6 @@
 CURRENT_DIRECTORY = pathlib.Path(__file__).parent.absolute()
 
 nox.options.sessions = [
-    "unit-3.9",
     "unit-3.10",
     "unit-3.11",
     "unit-3.12",